Control unit, method and computer-readable medium for operating a ventilator

ABSTRACT

To set trigger conditions correctly in pneumatic mode, a ventilator is controlled to obtain a measurement value of a bioelectric signal representative of the patient&#39;s breathing function, determine, based on the bioelectric signal, at least one point in time at which the patient starts inhalation, obtain a measurement value to be used for triggering an inspiration phase in the ventilator during the at least one point in time, determine a trigger condition for the inspiration phase on the basis of the measurement value, and use the trigger condition for initiating inspiration when ventilating the patient in support mode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a ventilator for use in support modeand a method for ventilating a patient in support mode.

2. Description of the Prior Art

A ventilator for providing breathing support to a patient can work indifferent modes, depending, i.e. on the patient's condition. If thepatient is showing some breathing activity a support mode is oftensuitable, in which the ventilator provides extra breathing support inphase with the patient's own breathing activity. In this case thepatient's own breathing activity must be monitored in an appropriate wayin order to synchronize the breathing support provided by the ventilatorwith the patient's own breathing so that an inspiration phase is startedby the ventilator when the patient starts inhaling. Typically apneumatic trigger condition based on pressure and/or flow in theventilator is set.

In some cases it is difficult to synchronize the ventilation correctlywith the patient's breathing efforts. For example, if there is aleakage, it will be difficult to set a suitable pneumatic trigger level.Finding a suitable level may require a lot of trial and error. Inparticular, when ventilating children leakages generally occur, since inthat case a cuff is typically not used around the tube.

Imperfect synchronization between the ventilator's and the patient'sbreathing cycles can lead to increased work for the patient. If thetrigger is too insensitive an entire breath may be skipped. If thetrigger is too sensitive an inspiration may be triggered in theventilator when the patient is not ready to inhale.

SUMMARY OF THE INVENTION

It is an object of the invention to improve the trigger conditions whenventilating a patient in support mode

This object is achieved according to the present invention by a computerreadable medium having stored thereon computer readable code forcontrolling a ventilator providing breathing support in a support modeto a patient code including computer readable instructions which, whenrun in a control unit controlling a ventilator will cause the controlunit to

-   -   obtain a measurement value of a bioelectric signal        representative of the patient's own breathing function,    -   determine, based on the bioelectric signal, at least one point        in time in which the patient starts inhalation,    -   obtain a measurement value to be used for triggering an        inspiration phase in the ventilator during said at least one        point in time,    -   determine a trigger condition for the inspiration phase on the        basis of the measurement value, and    -   use the trigger condition to for initiating inspiration when        ventilating the patient in support mode.

The object is also achieved by a method of controlling a ventilatorproviding breathing support in a support mode to a patient,characterized by the steps of

-   -   measuring a bioelectric signal representative of the patient's        own breathing function;    -   determining, based on the bioelectric signal, at least one point        in time in which the patient starts inhalation,    -   measuring the pressure in the ventilator during said at least        one point in time,    -   determining a trigger condition for the inspiration phase on the        basis of the measured pressure.

According to the invention, the Edi signal is used to determine thepoint in time when the patient starts inhaling, and the triggeringconditions for an inspiration phase in the ventilator may be adjustedbased on measurements of pressure and/or flow performed in theventilator at this point in time. The invention therefore facilitatesventilation in pneumatic mode that is adapted to the patient's ownbreathing cycle.

As will be understood by those skilled in the art, the method isperformed by a computer program, preferably located in the control unitof the ventilator for controlling the ventilator. Hence, the inventionalso relates to a control unit for a ventilator comprising a computerprogram product as defined above and a ventilator having such a controlunit.

Preferably, the control unit will be caused to adjust the triggercondition by an amount determined on the basis of the measurement value.

In one embodiment the encoded instructions the control unit to determinethe trigger condition on the basis of several measurement values, eachobtained at a point in time when the patient starts inhalation, indifferent breaths. This will provide a more accurate value for thetrigger condition.

In a preferred embodiment, the computer readable instructions which,when run in a control unit controlling a ventilator will cause thecontrol unit, after determining the start of inhalation and beforemeasuring the pressure, to determine whether an inspiration phase in theventilator was triggered before the start of inhalation and, if so, toslightly delay the ventilator's inspiration phase until the patient'sown breathing attempt can be detected.

In one embodiment the code instructions cause the control unit to adjustthe trigger condition incrementally in such a way as to reduce the timedifference between start of the ventilator's inspiration phase and thestart of the patient's inspiration. The trigger condition may beadjusted, for example, in fixed increments or in increments determinedon the basis of the difference between the measurement value and thetrigger condition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a ventilator providing breathing support to apatient.

FIG. 2 illustrates a situation where the trigger condition is toosensitive.

FIG. 3 illustrates a situation where the trigger condition is tooinsensitive.

FIG. 4 is a flow chart of an embodiment of the inventive method.

FIG. 5 is a more detailed flow chart of one of the steps of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic overview of a patient 1 connected to a ventilator3. The ventilator is arranged to work in support mode but can also bearranged to work in a controlled mode. To capture the Edi signal, thepatient 1 has an oesophageal catheter 5 inserted in order to record amyoelectric signal from the diaphragm. This myoelectric signal (EMGsignal) is fed to a control input 7 of the ventilator 3 and processed ina control unit 9 in the ventilator to produce the overall signal, calledan Edi signal. According to the invention, a bioelectric signal relatedto breathing, such as the Edi signal is used to adjust the pneumatictriggering criteria of the ventilator.

Typically the ventilator has registration unit 11 for monitoring thepressure and/or flow of breathing gas in the breathing circuit 1. Thecontrol unit 9 has a processor and at least one computer program that isexecuted to compare the patient's own breathing activity to thebreathing support provided by the ventilator and to adjust, ifnecessary, the triggering of the inspiration to synchronize it betterwith the patient's own breathing. This will be discussed in more detailbelow.

In order to assist the patient's breathing in such a way that thepatient's own attempts to inhale air are supported by an additional flowof breathing gas from the ventilator, the flow and/or pressure in theventilator is measured. The pressure and/or flow sensors in theventilator are used together with the trigger settings in order todetect the patient's attempt to inhale and initiate the inspirationphase. This is referred to as pneumatic triggering. Alternatively, anincreased flow in the direction towards the patient is measured. Ideallythe start of this inspiration phase should be perfectly synchronizedwith the start of the patient's own inhalation. To achieve this, thethreshold value for the flow and/or pressure must be set correctly sothat the flow and/or pressure measured in the ventilator will pass thethreshold at exactly the same time as the patient starts inhaling. Thisis not always the case, as will be discussed in the following.

FIG. 2 illustrates a situation in which the pneumatic trigger functionis based on pressure measurements and is too sensitive, causing theinspiration to be triggered too early in the breathing cycle compared tothe patient's own breathing. Three curves are shown varying along a timeaxis denoted t. The solid curve represents the Edi signal recorded inthe patient, that is, it reflects the patient's own breathing activity.The positive flank represents an inhalation by the patient. The dashedcurve is an ideal ventilator cycle, starting inspiration (positiveflank) when the patient starts inhaling. The dotted curve is an exampleof the breathing support that will result if the trigger condition istoo sensitive. In this case, the triggering should be made to startlater to be in phase with the patient's own breathing.

Too early triggering, as illustrated in FIG. 2, may be caused, forexample, by a leakage in the breathing circuit or if there is water inthe tubes. It may also be due to oscillations caused by variations inthe patient's thorax, caused by heart activity. In the case shown inFIG. 2, the pressure will drop below the trigger condition at a firstpoint in time t1, which occurs before the patient actually startsinhaling, at a second point in time t2. Hence, the ventilator will startinspiration support before the patient is ready to inhale. In this case,therefore, the pneumatic triggering should be delayed to be in phasewith the patient's own breathing.

FIG. 3, like FIG. 2, illustrates a situation in which the pneumatictrigger function is based on pressure measurements. In FIG. 3, thetrigger function is not sensitive enough, causing the inspiration to betriggered too late in the breathing cycle compared to the patient's ownbreathing. Three curves are shown varying along a time axis denoted t.As in FIG. 2, the solid curve represents the Edi signal recorded in thepatient, that is, it reflects the patient's own breathing activity. Thepositive flank represents an inhalation by the patient. The dashed curveis an ideal ventilator cycle, starting inspiration (positive flank) whenthe patient starts inhaling. In FIG. 3, the dotted curve is an exampleof a delayed triggering that will result if the trigger condition is tooinsensitive. In this case, when the patient starts inhaling, at a pointin time t3, the pressure will drop but not enough to trigger aninspiration phase in the ventilator at once. Only at a second point intime t4 will the ventilator start its inspiration phase. Hence, therewill be a time delay td between the point in time t3 when the patientstarts to inhale and the point in time t4 when the ventilator starts aninspiration.

In both the cases illustrated in FIGS. 2 and 3, the correct triggeringpoint in time, that is the point in time when the patient starts toinhale, can be determined by means of an Edi signal recorded on thepatient. By monitoring the Edi signal, the point in time when thepatient starts to inhale can be determined, as the start of the positiveflank of the Edi signal shown in FIGS. 2 and 3.

A first preferred embodiment of the inventive method is shown in FIG. 4.In this embodiment, as well as FIGS. 2 and 3 above, the triggering isbased on pressure measurements. Those skilled in the art can easilymodify this to triggering on flow criteria instead, or on a combinationof flow and pressure criteria, if the ventilator supports this.

To initiate the method, in step S41, the Edi signal is monitored duringat least one breath in the patient. In step S42, either the point intime when the Edi signal indicates patient inhalation during thisbreath, is determined, that is, the point in time in which the Edisignal raised above a certain predetermined value, or the point in timewhen the pneumatic trigger condition is reached, whichever occurs first.In step S43, preferably, it is determined if the ventilator is triggeredbefore the start of inhalation. If yes, the triggering has to bedelayed, in step S44. The point in time when the Edi signal indicatespatient inhalation is then determined in step S45. After step S45, orafter step S43 if the triggering was not too early, the pressure in theventilator at the starting time of inhalation is measured in step S46.This pressure, that is, the pressure at the actual start of inspirationby the patient, is used in step S46 as an indicator of what thepneumatic trigger condition should be. Finally, in step S47 the newtrigger condition is set to be used in the following breaths, orpresented to the operator as a proposed new setting.

The method may be performed during one breath only, or may be performedduring several breaths to obtain an average measured value. Such anaverage value will probably provide a more correct value of the pressurein the ventilator at the onset of the patient's own inspiration than ameasured value obtained during only one breath. In both cases, themethod may be performed again at certain time intervals to ensurecorrect timing of the breathing support. Alternatively, the proceduremay be performed again if the difference between the start of thebreathing cycle of the ventilator and that of the patient becomes toobig.

The adjustment procedure may be initiated by an operator. Instead ofautomatic adjustment, the operator can also use the result to adjust thetrigger condition manually, thereby adjusting the timing of thebreathing support cycle.

In step S44 the triggering of the ventilator should be delayed so as toenable correct measurement of the pressure and/or flow at the point intime when the patient starts to inhale. Therefore, the triggering shouldnot be performed until after the patient's inhalation has started.However, a maximum delay should be set, to ensure that the breathingsupport delay will not be harmful to the patient.

The correction of the trigger condition, based on the value determinedin step S47, may be carried out in different ways. The triggercondition, which, in the case of pressure triggering, will be a pressurevalue, which may be set as a function of the pressure measured in stepS46.

It may be favourable to adjust the trigger condition in several steps.In this case, step S47 will comprise the following substeps, illustratedin FIG. 5:

In step S51 comparing the pressure measured in step S43 to the actualtrigger condition currently applied in the ventilator .

In step S52 adjusting the trigger condition in the direction of themeasured pressure value. If the measured pressure value is lower thanthe pressure value that will trigger the inspiration phase, thethreshold value should be lowered. If the measured pressure value ishigher than the threshold pressure that will trigger the inspirationphase, the threshold value should be raised. The change in the thresholdvalue may, however, be carried out stepwise, so that the triggerconditions will be refined gradually. The steps could be carried out,for example, in fixed increments, for example, 0.1 cmH2O at a time, oras a fraction of the difference, for example 10% of the determineddifference each time. The procedure may be iterated a predeterminednumber of times, or until the difference between the trigger conditionsand the measured pressure is within an acceptable interval. This isindicated by decision step S53 in FIG. 5, which terminates the procedureif the difference is below a set limit and returns to step S51 if thedifference is still too large. Instead of determining the differencebetween actual pressure and threshold value, in the decision step S53the difference in time between the start of the patient's own inhalationand the start of the inspiration phase of the ventilator could beevaluated, that is, the difference between the first and second pointsin time t1 and t2, or the difference between the third and fourth pointsin time t3 and t4, as the case may be. In this case, if the timedifference is longer than a predetermined time, for example 100 ms, theprocedure of FIG. 5 should be reiterated. The predetermined time couldbe of the order of magnitude of 100 ms. It may be determined as a fixvalue, or based on duration of the patient's own breathing cycle, orinspiration phase.

Preferably, a pressure and/or flow interval is defined in which thetrigger condition can be set, to avoid setting the trigger condition toa value that may be harmful to the patient.

Also, in step S44 a maximum delay should be set for the pneumatictriggering to avoid losing an entire breath. This maximum delay couldbe, for example 300 ms. It could also be based on a measured duration ofthe patient's breathing cycle or inspiration phase. If no Edi triggeringhas occurred after the maximum delay, then the triggering value could beset to the value measured in the ventilator at the maximum delay andthis could be used as an initial value. If no breathing activity can bedetected from the Edi signal, a breath should still be delivered to thepatient within a suitable time.

A minimum pressure should be set, which will always trigger theventilator, even when the maximum delay has not been exceeded. Thisshould correspond to the least sensitive pressure that is allowed.

In order to evaluate the result of the adjustment, the time differencebetween the pneumatic control of the ventilator and the Edi signal maybe determined continuously or at certain time intervals. In this way thechanges in the time difference over time can be monitored andappropriate action can be taken when needed.

The Edi signal is prone to disturbances, for example, from strongerbioelectric signals in the patient's body. To avoid using an erroneousEdi signal as a basis for the trigger conditions, an automaticadjustment should not be allowed if the time difference betweeninspiration phase triggered by the ventilator and the patient's owninhalation is too great. Alternatively a quality indicator for the Edisignal could be used, to ensure that the Edi signal actually reflectsthe patient's breathing activity, and not an artefact.

Before starting the actual adjustment of the trigger conditions, byperforming the steps of FIG. 4, it may be useful to measure the Edisignal and the ventilator's breathing cycle for some breaths to comparethe timing of the two. This comparison will indicate if the inspirationsupport is triggered too early or too late compared to the patient's ownbreathing activity, thereby indicating in which direction the triggercondition should be adjusted. Such a comparison can also be performedcontinuously, or at certain time intervals, to evaluate the need foradjusting the trigger conditions. If the timing of the Edi signal andthe breathing support cycle differs less than a certain limit noadjustment is needed. If the difference in timing exceeds this limit anadjustment procedure as the one shown in FIG. 4 should be performed.

The difference between the inspired and expired volumes may be used toevaluate whether there is any leakage before the trigger condition isadjusted. If a considerable leak is present the sensitivity of theadjusted trigger should preferably be limited.

As mentioned above, the triggering may be based on pressure or flow ofgas in the ventilator. In the case of a leakage, pressure triggeringwill be more suitable than flow triggering, since a leakage will cause aflow, even if there is no patient activity.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

1-16. (canceled)
 17. A computer-readable medium encoded with programminginstructions, said medium being loaded into a computerized control unitof a ventilator that provides breathing support in a support mode to apatient, said programming instructions causing said computerized controlunit to: obtain a measurement value of a bioelectric signalrepresentative of the breathing function of the patient; determine,based on the bioelectric signal, at least one point in time at which thepatient starts inhalation; obtain a measurement value to be used fortriggering an inspiration phase in the ventilator during said at leastone point in time; determine a trigger condition for the inspirationphase based on said measurement value; and use said trigger condition toinitiate inspiration when ventilating the patient in said support mode.18. A computer-readable medium as claimed in claim 17 wherein saidprogramming instructions cause said control unit to determine saidtrigger condition based on a plurality of measurement values, eachobtained at a point-in-time when said patient begins inhalation forrespectively different breaths.
 19. A computer-readable medium asclaimed in claim 17 wherein said programming instructions cause saidcontrol unit to adjust said trigger condition by an amount determineddependent on said measurement value.
 20. A computer-readable medium asclaimed in claim 17 wherein said programming instructions cause saidcontrol unit, after determining the start of inhalation and beforeobtaining said measurement value, to determine whether an inspirationphase was triggered in the ventilator before said start of inhalationand, if so, to delay said initiating of inspiration until a spontaneousattempt by the patient to breath is detected.
 21. A computer-readablemedium as claimed in claim 17 wherein said programming instructionscause said control unit to adjust said trigger condition incrementallyto reduce a time difference between a start of an inspiration phase ofsaid ventilator and the start of inspiration by the patient.
 22. Acomputer-readable medium as claimed in claim 21 wherein said programminginstructions cause said control unit to adjust said trigger condition infixed increments.
 23. A computer-readable medium as claimed in claim 21wherein said programming instructions case said control unit to adjustsaid trigger condition in increments determined from a differencebetween said measurement value and said trigger condition.
 24. A controlunit for a ventilator operated by a computer-readable medium comprisingprogramming instructions, said programming instructions configuring saidcontrol unit to: obtain a measurement value of a bioelectric signalrepresentative of the breathing function of the patient; determine,based on the bioelectric signal, at least one point in time at which thepatient starts inhalation; obtain a measurement value to be used fortriggering an inspiration phase in the ventilator during said at leastone point in time; determine a trigger condition for the inspirationphase based on said measurement value; and use said trigger condition toinitiate inspiration when ventilating the patient in said support mode.25. A ventilator comprising: a breathing circuit adapted for connectionto a patient, said breathing circuit being operable in a support mode toassist breathing by the patient; a control unit that operates saidbreathing circuit, said control unit being configured to obtain ameasurement value of a bioelectric signal representative of thebreathing function of the patient, determine, based on the bioelectricsignal, at least one point in time at which the patient startsinhalation, obtain a measurement value to be used for triggering aninspiration phase in the ventilator during said at least one point intime, determine a trigger condition for the inspiration phase based onsaid measurement value, and use said trigger condition to initiateinspiration when ventilating the patient in said support mode.
 26. Amethod for operating a computerized control unit of a ventilator thatprovides breathing support in a support mode to a patient, said methodcomprising the steps of: obtaining a measurement value of a bioelectricsignal representative of the breathing function of the patient; in aprocessor, determining, based on the bioelectric signal, at least onepoint in time at which the patient starts inhalation; obtaining ameasurement value to be used for triggering an inspiration phase in theventilator during said at least one point in time; in said processor,determining a trigger condition for the inspiration phase based on saidmeasurement value; and from said processor, using said trigger conditionto initiate inspiration when ventilating the patient in said supportmode.
 27. A method as claimed in claim 26 comprising determining saidtrigger condition based on a plurality of measurement values, eachobtained at a point-in-time when said patient begins inhalation forrespectively different breaths.
 28. A method as claimed in claim 26comprising, in said processor, adjusting said trigger condition by anamount determined dependent on said measurement value.
 29. A method asclaimed in claim 26 comprising, after determining the start ofinhalation and before obtaining said measurement value, determiningwhether an inspiration phase was triggered in the ventilator before saidstart of inhalation and, if so, delaying said initiating of inspirationuntil a spontaneous attempt by the patient to breath is detected.
 30. Amethod as claimed in claim 26 comprising, in said processor, adjustingsaid trigger condition incrementally to reduce a time difference betweena start of an inspiration phase of said ventilator and the start ofinspiration by the patient.
 31. A method as claimed in claim 30comprising adjusting said trigger condition in fixed increments.
 32. Amethod as claimed in claim 30 comprising adjusting said triggercondition in increments determined from a difference between saidmeasurement value and said trigger condition.